Add link to Help Center article about moderators in the moderators tab on the /users page

The moderators tab on the /users page has some information and links about moderating/moderators on Stack Exchange. I think it would make sense to add a link to the main Help Center article about moderators: Who are the site moderators, and what is their role here? - part of that content is already in the A Theory of Moderation blog post, but that has been updated/expanded and may be improved more in the future, while the blog post is not.